Plastic injection molding warpage

There is a law for warpage but unfortunately it does not take you much further forward. It is that warping is proportional to internal stress in the molded plastic part. There is always going to be a lot of that because conventional plastic injection molding is a forceful process thanks to the very high viscosity of the plastics melt. It involves, inter alia, high pressures on compressible materials, high shear stresses on long tangled molecules, large temperature gradients in conflict with poor thermal conductivity, and movement constraints applied by some but not all parts of the plastic injection mold. It is allied to shrinkage: indeed many authorities have defined warpage as non-uniform shrinkage. So within your particular scenario of product geometry, material choice, injection mold design and process conditions, it will help to think about ways of reducing shrinkage magnitude and variation. Which element will have the major influence depends so much on the scenario, which is why the general law does not help much.

You seem to be on exactly the right lines by using DOE and simulation to examine the strategies that remain open to you after the constraints imposed by the rest of the supply chain. Having done that, you can state with a confidence of perhaps 75-80 percent, what the best outcome is likely to be. It may or may not be what people want to hear. If people then want to sanction the high cost and delay of physical experiments instead of simulation, you will be able to up the confidence level of your statements.

By the way, don’t let people make you think you are responsible for giving them what they want. You are responsible for telling them what is feasible, even if it is bad news. And the corollary of bad news should be a rethink of the constraints.

You could perhaps consider the MuCell process. This drastically reduces internal stress in the plastic injection mold molding by two means. First, the viscosity of the melt is greatly reduced and second, the high-pressure pack phase is eliminated. It is possible to use a conventional injection mold with MuCell if it is valve-gated but overall shrinkage is likely to be reduced, so there could be dimensional problems.

There are different causes of warpage and different solutions to that problem, and all depend mostly on the shape of part design and on type/deformation of warp. From my experience 80% of warp problems are related to plastic part design. Warp can be addressed and solved (eliminated or significantly reduced) by proper part design for the particular plastic material. About 15% of warpage is related to tool design and the rest to IM process. All three element (design, tool, process) must be correct for particular material and if any of those elements are improper then warp occurs.

- - - -> by: Andrew

"Best practice" approach to curing warpage on existing mold I would suggest the following:1. run a warpage analysis of the tool on mold flow.2. you will probably find that your mold flow predictions are quite different than what you actually get in the field.3. now you need to "calibrate" the analysis until you get warpage results similar to those you are getting in the field. this may include changing the simulation ,material, manually inserting shrinkage measured data or thermal data and manual correction of the drawings used to include such elements as surface patterns and molder mistakes.4. once you get simulation results similar to actual results, only then can you begin to work on solving the problem and the solution will change from part to part and from mold to mold.

- - - -> by: Daniel

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